In conventional radiography, image quality and diagnostic value can be compromised when the object density differs drastically between different parts of an object. For example, in conventional chest x-rays the mediastinum and retrocardiac area can be underexposed, detracting from the diagnostic value of the image. In fact, many diagnostic errors in chest radiography occur in areas displayed with suboptimal contrast or penetration. Missed lung metastases, for example, can reside in these underexposed areas, camouflaged from view.
Equalization is a common term in the x-ray radiography field which refers to the process of selectively attenuating certain portions of the patient or object exposed to the x-ray. An x-ray radiographic image can be equalized by selectively attenuating only those areas of the image that are determined to have been overexposed. The effect of equalization is to reduce the intrinsically large dynamic range of the x-ray beam intensities in order to accommodate the dynamic range limitations of most x-ray detector systems. The most common detector systems employed in diagnostic x-ray radiography are film and image intensifier-TV systems, both of which have severely limited dynamic ranges.
Different methods have been presented to correct the problems described above. For example, one method involves arranging a plurality of filters between an x-ray emitter and the image receptor. The filters are selected and arranged so that only the areas of over-exposure are attenuated. Practice of this method provides acceptable results, once the correct combination of filters are found. However, a serious drawback of this method is that it is cumbersome since filter selection and juxtaposition is a manual process and can require time consuming trial and error for the correct combination to be found.
Another approach for addressing the above-described problem involves variably attenuating different portions of an x-ray beam by placing an attenuating material between an x-ray source and a patient. For example, one method is described in Peppler, et al., "Digitally Controlled Beam Attenuator," SPIE, 347:106-111, 1982. Peppler describes a digital beam attenuating device for attenuating specific areas of an imaged object. The device consists of an attenuation chamber which is placed between the x-ray tube and the patient. The chamber consists of a 6.times.6 array of 1 cm square lucite pistons each with a 1 cm stroke. An attenuating material of cerium chloride (CeCl.sub.3) in solution is housed within a shapable latex membrane and placed in the path of the x-ray beam between the x-ray tube and the patient. A computer controls the advancement of the pistons which press into the membrane, adjusting the thickness of the CeCl.sub.3 at certain cells. The degree of attenuation depends on the thickness of the CeCl.sub.3. A solenoid controls water lines which control the advancement and retreat of the pistons. A reservoir is coupled to the latex membrane to catch overflowing solution when the membrane is compressed.
The most critical limitation with Peppler is that the empty digital beam attenuator reduces the primary x-ray attenuation by a factor of 15, which creates severe tube loading problems. Furthermore, Peppler utilizes a large number of dynamic elements such as the water lines and solenoids. These increase the chances of breakdowns, thereby increasing the costs of maintenance.
Another method is described in U.S. Pat. No. 4,497,062 to Mistretta (and a corresponding article: Hasegawa, B. H., Dobbins III, J. T., et al., "Feasibility of Selective Exposure Radiography," SPIE, 454:271-278, 1984). Mistretta describes a digitally-controlled x-ray beam method and apparatus. The apparatus comprises a computer which analyzes an initial x-ray image to locate high and low gray level regions. Based on this determination, an attenuation number for each pixel in the image is assigned and transmitted to a printer; such as a dot matrix or ink jet printer. A non-attenuating substrate is fed through the printer while the printer prints an attenuating pattern onto the substrate. The "ink" present on the ribbon contains attenuating material such as cerium oxide. The printer deposits the attenuating cerium in varying thicknesses depending on the attenuation number. The substrate is then placed between the x-ray source and the patient during a subsequent regular x-ray.
A major limitation of the technique proposed by Mistretta is the relatively long time required for custom fabrication of the compensation filter that precluded its clinical implementation. Specifically, it is described that the printing time of 80 layers of a 64.times.64 pixel attenuator, as of the publication date of the article, was less than 30 minutes. It was hoped that by increasing the number of print heads and by using faster repetition rates, the time could be reduced to 30 seconds. This is still not acceptable for clinical applications because a patient must remain absolutely still between the time the initial scan is performed, during the creation of the attenuator, and through the final scan. If the patient was to move, even slightly, the attenuation pattern would change and the image would not be properly attenuated.
What is needed is a simple device that can variably attenuate different regions of an x-ray signal in order to provide a clearer x-ray image of an object having variable densities.